This invention relates to turbines, impellers, propellers, and the like, and particularly to propellers for water craft.
The search for propellers having low cost and yet good performance is ongoing. As described in U.S. Pat. No. 6,371,726 to C. Jonsson et al., the general design goal of a propeller is high performance, i.e., high forward thrust or propeller efficiency at any speed. One approach to this goal is a large propeller diameter in combination with a low drive-shaft speed, with blades having optimal radial (hub to tip) load distributions, areas large enough to avoid cavitation, and thin cambered sections of the airfoil type.
Traditional materials used for propellers for marine applications, such as steel, aluminum, and bronze, provide good strength and stiffness but now can be more expensive than newer composite and plastic or polymer materials that have been used in propellers for some time. Nevertheless, the performance of such newer materials in applications like the marine application has generally been poor.
Some composite materials, such as hand-laid fiber-reinforced composites and resin-transfer-molded composites, have shown promise, but they are so expensive that they can cost more than a molded aluminum propeller. The flexural strength of composites and polymers also is often not high enough to obtain performance equivalent to a metal propeller. Plastic polymer or plastic-composite propellers may have the required strength, but they often do not have the stiffness needed to replace metals like aluminum with equivalent performance. Because composites deflect under load, the performance of a composite propeller can suffer because its shape can differ from the optimal shape.
A useful goal is a propeller or a propeller blade, which is part of a propeller assembly, that is made of a light and flexible material and that yet performs substantially the same as propellers or propeller blades made of stiffer materials, for example, aluminum. Prior attempts to reach this goal have been unsuccessful.
European Patent Publication EP 0 295 247 discloses a propeller made of an expensive hand-laid composite polymer-matrix material. The propeller is elastically deformable, and thus the pitch, which is the distance a cylindrical section of the propeller ideally moves in one rotation, varies under load. The pitch is controlled by carefully making the propeller stronger or weaker at predetermined places on the blades. A beam is used to support a propeller blade in the radial or span-wise direction of the blade, thus providing additional strength and resistance to bending in that direction.
Patent Abstracts of Japan Publications No. JP 11-314598 and No. JP 11-180394 describe propellers made from reinforced resin materials that allow the propellers' pitch to change under load. Publication No. JP 11-314598 describes strengthening propeller blades in certain directions by suitably orienting the reinforcing fibers, and although it mentions blade deflections, the Publication does not address the issues of camber, pitch, and optimum pitch to get optimum performance.
U.S. Pat. No. 3,318,388 to Bihlmire discloses a metal/plastic composite propeller, where the plastic is molded over the metal, that allows the propeller's pitch to alter under load. Other propellers made from polymer materials are described in U.S. Pat. No. 5,275,535 and No. 4,842,483 and European Patent Publication No. EP 0 254 106.
None of the above-cited documents discusses any particular combination of pitch distribution and camber profile of a flexible propeller or propeller blade that enables the propeller or blade to deflect into an optimum design pitch distribution at design and off-design conditions.